As cellular systems of this type, various systems have conventionally been proposed. Recently, as one of the cellular systems of this type, the cellular system in the form of a high-speed downlink packet transmission (HSDPA: High-Speed Downlink Packet Access) system has been proposed by the 3GPP (3rd Generation Partnership Project).
Although description will be given hereinbelow about an example where the foregoing HSDPA cellular system is used as a cellular system that carries out packet communications, the present invention is not limited to the HSDPA.
In the HSDPA system, a high-speed downlink shared channel (HS-PDSCH: High-Speed Physical Downlink Shared Channel) is used for transmitting high-speed data in downlink from a base station to mobile stations in the cellular system and further for transmission in downlink from the base station to the mobile stations. This HS-PDSCH is used for data transmission from each base station to a plurality of mobile stations and, for this purpose, each base station or its control station determines a schedule for performing data transmission to the respective mobile stations to thereby transmit data at timings that differ per mobile station (time-division system).
In order to control such data transmission from each base station to the mobile stations, each base station sets a DPCH (Dedicated Physical Channel) being a dedicated channel with respect to each of the plurality of mobile stations. This DPCH is used for transmitting control information by its downlink signal from the base station to the mobile station and for transmitting a control signal by its uplink signal from the mobile station to the base station.
In each mobile station, although the ratio of time for receiving data using the HS-PDSCH is small, the DPCH continues to be set with respect to the base station even in a data waiting state where data is not received, so that transmission of data can be started in a short time after requesting the transmission of data. Therefore, although each base station performs data transmission with respect to only one mobile station at the same time, many mobile stations are in the data waiting state and set DPCHs with respect to the base station.
On the other hand, in the cellular system, there is a technique called soft handover where a mobile station sets channels with respect to a plurality of base stations at the same time. Each base station transmits a common pilot signal at a predetermined power, and a mobile station sets a DPCH with respect to such a base station of which the reception power of the common pilot signal is the greatest. On the other hand, in the soft handover, if there exists another base station of which the reception power of the common pilot signal has only a small difference, the mobile station also sets a DPCH with respect to such another base station, resulting in setting the DPCHs with respect to the plurality of base stations. In the following description, those base stations with the DPCHs set in the soft handover will be called connection base stations.
Further, in the cellular system, a technique called a high-speed closed-loop transmission power control is applied. This high-speed closed-loop transmission power control is applied to a DPCH with respect to both its uplink and downlink. In the DPCH uplink transmission power control, a base station uses a dedicated pilot signal included in an uplink signal to measure its reception SIR (Signal to Interference Ratio) and compares between a measured value thereof and a predetermined target SIR. Then, the base station makes notification to a mobile station by including TPC (Transmit Power Control) bits indicative of power increase in a DPCH downlink signal when the measured value is smaller than the target SIR, and otherwise, TPC bits indicative of power decrease. Then, the mobile station receives the TPC bits and, depending on the TPC bits, increases or decreases the transmission power.
In case of using this uplink transmission power control along with the soft handover, the mobile station receives TPC bits from each of a plurality of connection base stations and, when at least one set of the TPC bits is indicative of power decrease, the mobile station decreases the DPCH transmission power, and otherwise (i.e. when all TPC bits are indicative of power increase), increases the DPCH transmission power. By implementing such a transmission power control, the reception quality of the uplink signal satisfies the target SIR in at least one connection base station, and simultaneously, the reception qualities of the uplink signals are prevented from exceeding the target SIR in all connection base stations, to thereby prevent the uplink interference wave power from increasing.
On the other hand, in the DPCH downlink transmission power control, a mobile station uses a dedicated pilot signal included in a downlink signal to measure its reception SIR and compares between a measured value thereof and a predetermined target SIR. Then, the mobile station makes notification to a base station by including TPC bits indicative of power increase in a DPCH uplink signal when the measured value is smaller than the target SIR, and otherwise, TPC bits indicative of power decrease. Then, the base station receives the TPC bits and, depending on the TPC bits, increases or decreases the transmission power.
In case of using this downlink transmission power control along with the soft handover, the mobile station receives DPCH downlink signals respectively from a plurality of connection base stations and combines them together, and compares a reception SIR of a downlink signal after the combination with a target SIR to determine TPC bits. Then, the mobile station transmits the common TPC bits to the plurality of connection base stations and, depending on the TPC bits, the connection base stations respectively increase or decrease the transmission power. By causing all connection base stations to increase or decrease the transmission power according to the common TPC bits as described above, the balance is kept in transmission power among the connection base stations so that the downlink signal transmitted from such a base station that causes the minimum propagation loss between itself and the mobile station is received at the mobile station with excellent quality, thereby preventing the transmission power of the downlink signals from increasing more than necessary to thus prevent the downlink interference wave power from increasing.
The transmission power control and the soft handover as described above have become an effective technique as a radio access system, particularly for reducing the transmission power to decrease the interference wave power to thereby increase the link capacity in the CDMA (Code Division Multiple Access) cellular system.
Here, during the soft handover, as the system is schematically shown in FIG. 10, a mobile station (MS) 3 has DPCHs simultaneously connected to a plurality of connection base stations (BS) 1 and 2 as described before, but an HS-PDSCH is connected to only one base station (base station 1 in the figure). Further, as described before, it is necessary for the mobile station 3 to transmit to the base station reception confirmation notification (ACK/NACK: Acknowledge/Non-Acknowledge) information indicative of whether or not the mobile station 3 has received packets transmitted by the HS-PDSCH without error, and this reception confirmation notification information is transmitted by using an uplink HS-DPCCH (High-Speed Dedicated Physical Control Channel: HS-PDSCH dedicated control channel including packet transmission control information).
A relationship between the uplink HS-DPCCH and the uplink DPCH is as shown in FIG. 11. The DPCH is composed of a DPCCH (Dedicated Physical Control Channel) and a DPDCH (Dedicated Physical Data Channel). The DPCCH includes a dedicated pilot channel (Pilot), TPC bits, and an FBI (Feed Back Information). On the other hand, the DPDCH is data and includes user information and control information. The DPCCH and the DPDCH are mutually orthogonally modulated, multiplexed, and transmitted.
The HS-DPCCH is allocated a slot length corresponding to three slots of the DPCCH or DPDCH and includes the foregoing reception confirmation notification (ACK/NACK) information and a CQI (Channel Quality Indicator) indicative of a downlink quality. The HS-DPCCH is code-multiplexed with the DPCH and transmitted. Since judgment on the ACK (receipt)/NACK (non) signal included in the HS-DPCCH is made only by the base station transmitting the HS-PDSCH, i.e. the packet transmission base station, diversity combination thereof between the base stations is not carried out.
On the other hand, as shown in FIG. 10, the uplink DPCHs are subjected to diversity combination between the connection base stations 1 and 2 in a radio network control station (RNC) 10. The DPCH is controlled to achieve a predetermined reception quality by the high-speed closed-loop transmission power control as described above, and the HS-DPCCH is transmitted at a transmission power PH which is the sum of a transmission power PD of the DPCH and a predetermined offset power Δ added thereto. That is, the HS-DPCCH is transmitted in a relationship ofPH=PD+Δ  (1).
In this manner, based on the ACK/NACK transmitted from the mobile station by using the HS-PDCCH, the packet transmission base station judges whether or not packets are correctly received by the mobile station. If judged to be NACK, the packet transmission base station deems that the corresponding packets are not correctly received by the mobile station, and retransmits the packets in question to thereby prevent a packet loss.
Here, particularly, if NACK is taken for ACK, although packets are not correctly received, the subject base station transmits next packets, and therefore, those packets not correctly received are not retransmitted and thus are lost in the mobile station, resulting in occurrence of a packet loss. Therefore, it is necessary that the reception error rate for NACK be sufficiently smaller than that for ACK, or, in other words, it is necessary that the reception quality of the ACK/NACK signal at the packet transmission base station be sufficiently high.
However, unless any measure is taken, the reception quality of ACK/NACK is lowered due to operation as described hereinbelow.
During the execution of the soft handover, the transmission power of the UL (Up-Link) DPCH serving as a reference of the transmission power of the HS-DPCCH is controlled in the following manner. When a reception SIR of the UL DPCH is greater than a reference SIR, the connection base station transmits a TPC signal that reduces the power, while, when the reception SIR after combination is smaller than the reference SIR, it transmits a TPC signal that increases the power.
On the other hand, the mobile station in the execution of the soft handover receives the TPC signal from each connection base station and increases the transmission power when all TPC signals are indicative of power increase, while decreases the transmission power when at least one of the TPC signals is indicative of power decrease. In this event, even if the reception SIR of the UL DPCH of the packet transmission base station is less than the reference SIR, when the reception SIR of the UL DPCH of the base station other than the packet transmission base station is equal to or more than the reference SIR, it represents that at least one of the TPC signals to the mobile station is indicative of power decrease. Consequently, the mobile station decreases the UL DPCH transmission power so that the reception SIR of the UL DPCH of the packet transmission base station is further reduced. Thus, the reception quality of the HS-DPCCH is lowered.
As described above, there is a problem that, during the soft handover, the reception quality of the HS-DPCCH is lowered due to the TPC signals of other than the packet transmission base station so that the ACK/NACK reception error is liable to occur.
During the execution of the soft handover, the transmission power of the DL (Down-Link) DPCH including the TPC signals for controlling the UL DPCH transmission power is controlled in the following manner. The mobile station during the execution of the soft handover combines DL DPCHs transmitted from the respective connection base stations and transmits a TPC signal that decreases the power when a reception SIR after the combination is greater than a reference SIR, while transmits a TPC signal that increases the power when the reception SIR after the combination is smaller than the reference SIR, and each base station controls the transmission power according to that TPC signal.
In this event, even if the reception SIR of the DL DPCH of the packet transmission base station is smaller than the reference SIR, when the reception SIRs of the DL DPCHs of the base stations other than the packet transmission base station are greater, the reception SIR after the combination becomes greater than the reference SIR. Consequently, each base station decreases the DL DPCH transmission power so that the reception SIR of the DL DPCH of the packet transmission base station is further reduced. Thus, the reception quality of the TPC signal transmitted from the packet transmission base station is lowered so that the TPC signal reception error increases. In this event, even if the packet transmission base station transmits a TPC signal that increases the transmission power of the mobile station for increasing the reception SIR of the UL DPCH, there is a problem that the reception SIR of the UL DPCH is lowered due to an error of that TPC signal and the reception quality of the HS-DPCCH is lowered accordingly so that the ACK/NACK reception error is liable to occur.
Particularly, due to occurrence of the TPC signal reception error in each base station, the transmission power of the DL DPCH of the packet transmission base station becomes lower than the transmission power of the DL DPCH of each of the other base stations, and therefore, the foregoing problem is liable to occur.
As conventional methods for improving the foregoing problems, the following two methods may be considered.
As the first method, there is a method where when a mobile station is in the soft handover state, the offset power Δ of the HS-DPCCH is increased as compared with the non soft handover state. In this method, there is a drawback that since a proper Δ differs depending on the number of connection base stations, the UL reception quality of a link transmitting packets, and so forth, it is difficult to determine a proper increment of Δ. Further, there is a drawback that since it is necessary to increase the value of Δ with a margin in order to sufficiently reduce the ACK/NACK reception error rate, the uplink transmission power increases more than necessary so that the power consumption of the mobile station increases and simultaneously the uplink interference wave power increases.
Moreover, there is a drawback that since the transmission power of the HS-DPCCH increases as compared with the DPCCH, the reception quality of the pilot signal of the DPCCH is degraded so that the channel estimation accuracy is degraded. It may be considered to add a pilot signal to the HS-DPCCH in order to clear the foregoing drawback. However, there is a problem that since the pilot signal that is used only during the soft handover is constantly transmitted, the use efficiency of resources is reduced.
As the second method, there is a method where, during the soft handover (SHO), a packet transmission base station produces a signal TPC-HS for a high-speed closed-loop control type transmission power control for the reception quality of the HS-DPCCH and transmits it instead of the normal TPC signal or both to a mobile station. In this method, in order to satisfy qualities of both the ACK signal and the NACK signal, the packet transmission base station must set a target SIR for the one that is required a stricter quality. Therefore, there is a drawback that excessive transmission signals are consumed. Further, there is also a drawback that, in case of transmitting the TPC-HS instead of the normal TPC, the power of the HS-DPCCH increases to cause interference and consequently the DPCH channel estimation accuracy of other channels is degraded. Moreover, there is a drawback that, in case of transmitting the TPC-HS apart from the normal TPC, it is necessary to change a slot format.
It is an object of the present invention to provide a cellular system that can receive with a high quality in a base station an ACK/NACK signal transmitted from a mobile station to the base station for notification of packet reception confirmation, while solving the foregoing various conventional problems.